Most accurate way that is somewhat affordable is with a salinity refractometer. It is a handheld device with a prism you place water on and can read the salt level in the water. You can also buy a swing arm hydrometer or even a thermometer that is a hydrometer for cheaper, but these aren’t as accurate. You want to keep you salinity level between 1.024 and 1.026. You control this with water top offs. Keep in mind that the salt does not evaporate from the tank like water does. As water evaporates the salt concentration in the water rises until it is topped off with freshwater to dilute it back to what it should be at. If your salinity is too high you bring it down over the period of a few days by taking water out and replacing it with freshwater. If salinity is too low just top off with saltwater until it gets to the correct level. Once your salinity is correct, always top off with freshwater, never salt.
